We present new images of Arp 220 from the Atacama Large Millimeter/submillimeter Array with the highest combination of frequency (691 GHz) and resolution ($0.36 times 0.20^{primeprime}$) ever obtained for this prototypical ultraluminous infrared gala
xy. The western nucleus is revealed to contain warm (200 K) dust that is optically thick ($tau_{434mu m} = 5.3$), while the eastern nucleus is cooler (80 K) and somewhat less opaque ($tau_{434mu m} = 1.7$). We derive full-width half-maximum diameters of $ 76 times le 70$ pc and $123 times 79$ pc for the western and eastern nucleus, respectively. The two nuclei combined account for ($83 ^{+65}_{-38}$ (calibration) $^{+0}_{-34}$ (systematic))% of the total infrared luminosity of Arp 220. The luminosity surface density of the western nucleus ($ log(sigma T^4) = 14.3pm 0.2 ^{+0}_{-0.7}$ in units of L$_odot$ kpc$^{-2}$) appears sufficiently high to require the presence of an AGN or a hot starburst, although the exact value depends sensitively on the brightness distribution adopted for the source. Although the role of any central AGN remains open, the inferred mean gas column densities of $0.6-1.8 times 10^{25}$ cm$^{-2}$ mean that any AGN in Arp 220 must be Compton-thick.
We present a multiwavelength study of a sample of far-infrared (FIR) sources detected on the Herschel broad--band maps of the nearby galaxy M33. We perform source photometry on the FIR maps as well as mid-infrared (MIR), H$alpha$, far-ultraviolet and
integrated HI and CO line emission maps. By fitting MIR/FIR dust emission spectra, the source dust masses, temperatures and luminosities are inferred. The sources are classified based on their H$alpha$ morphology (substructured versus not-substructured) and on whether they have a significant CO detection ($S/N>$3$sigma$). We find that the sources have dust masses in the range 10$^2$-10$^4$~M$_odot$ and that they present significant differences in their inferred dust/star formation/gas parameters depending on their H$alpha$ morphology and CO detection classification. The results suggests differences in the evolutionary states or in the number of embedded HII regions between the subsamples. The source background--subtracted dust emission seems to be predominantly powered by local star formation, as indicated by a strong correlation between the dust luminosity and the dust-corrected H$alpha$ luminosity and the fact that the extrapolated young stellar luminosity is high enough to account for the observed dust emission. Finally, we do not find a strong correlation between the dust-corrected H$alpha$ luminosity and the dust mass of the sources, consistent with previous results on the breakdown of simple scaling relations at sub-kpc scales. However, the scatter in the relation is significantly reduced by correcting the H$alpha$ luminosity for the age of the young stellar populations in the star--forming regions.
Data from the Herschel Space Observatory have revealed an unusual elliptical galaxy, NGC 4125, which has strong and extended submillimeter emission from cold dust but only very strict upper limits to its CO and HI emission. Depending on the dust emis
sivity, the total dust mass is 2-5x10^6 Msun. While the neutral gas-to-dust mass ratio is extremely low (< 12-30), including the ionized gas traced by [CII] emission raises this limit to < 39-100. The dust emission follows a similar r^{1/4} profile to the stellar light and the dust to stellar mass ratio is towards the high end of what is found in nearby elliptical galaxies. We suggest that NGC 4125 is currently in an unusual phase where evolved stars produced in a merger-triggered burst of star formation are pumping large amounts of gas and dust into the interstellar medium. In this scenario, the low neutral gas-to-dust mass ratio is explained by the gas being heated to temperatures >= 10^4 K faster than the dust is evaporated. If galaxies like NGC 4125, where the far-infrared emission does not trace neutral gas in the usual manner, are common at higher redshift, this could have significant implications for our understanding of high redshift galaxies and galaxy evolution.
The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey (NGLS) comprises an HI-selected sample of 155 galaxies spanning all morphological types with distances less than 25 Mpc. We describe the scientific goals of the survey, the sample select
ion, and the observing strategy. We also present an atlas and analysis of the CO J=3-2 maps for the 47 galaxies in the NGLS which are also part of the Spitzer Infrared Nearby Galaxies Survey. We find a wide range of molecular gas mass fractions in the galaxies in this sample and explore the correlation of the far-infrared luminosity, which traces star formation, with the CO luminosity, which traces the molecular gas mass. By comparing the NGLS data with merging galaxies at low and high redshift which have also been observed in the CO J=3-2 line, we show that the correlation of far-infrared and CO luminosity shows a significant trend with luminosity. This trend is consistent with a molecular gas depletion time which is more than an order of magnitude faster in the merger galaxies than in nearby normal galaxies. We also find a strong correlation of the L(FIR)/L(CO3-2) ratio with the atomic to molecular gas mass ratio. This correlation suggests that some of the far-infrared emission originates from dust associated with atomic gas and that its contribution is particularly important in galaxies where most of the gas is in the atomic phase.
Galactic cirrus emission at far-infrared wavelengths affects many extragalactic observations. Separating this emission from that associated with extragalactic objects is both important and difficult. In this paper we discuss a particular case, the M8
1 group, and the identification of diffuse structures prominent in the infrared, but also detected at optical wavelengths. The origin of these structures has previously been controversial, ranging from them being the result of a past interaction between M81 and M82 or due to more local Galactic emission. We show that over of order a few arcminute scales the far-infrared (Herschel 250 &mu&m) emission correlates spatially very well with a particular narrow velocity (2-3 km/s) component of the Galactic HI. We find no evidence that any of the far-infrared emission associated with these features actually originates in the M81 group. Thus we infer that the associated diffuse optical emission must be due to galactic light back scattered off dust in our galaxy. Ultra-violet observations pick out young stellar associations around M81, but no detectable far-infrared emission. We consider in detail one of the Galactic cirrus features, finding that the far-infrared HI relation breaks down below arc minute scales and that at smaller scales there can be quite large dust temperature variations.
An analysis of large-area CO J=3-2 maps from the James Clerk Maxwell Telescope for 12 nearby spiral galaxies reveals low velocity dispersions in the molecular component of the interstellar medium. The three lowest luminosity galaxies show a relativel
y flat velocity dispersion as a function of radius while the remaining nine galaxies show a central peak with a radial fall-off within 0.2-0.4 r(25). Correcting for the average contribution due to the internal velocitydispersions of a population of giant molecular clouds, the average cloud-cloud velocity dispersion across the galactic disks is 6.1 +/- 1.0 km/s (standard deviation 2.9 km/s), in reasonable agreement with previous measurements for the Galaxy andM33. The cloud-cloud velocity dispersion derived from the CO data is on average two times smaller than the HI velocity dispersion measured in the same galaxies. The low cloud-cloudvelocity dispersion implies that the molecular gas is the critical component determining the stability of the galactic disk against gravitational collapse, especially in those regions of the disk which are H2 dominated. The cloud-cloud velocity dispersion shows a significant positivecorrelation with both the far-infrared luminosity, which traces the star formation activity, and the K-band absolute magnitude, which traces the total stellar mass. For three galaxies in the Virgo cluster, smoothing the data to a resolution of 4.5 kpc (to match the typical resolution of high redshift CO observations) increases the measured velocity dispersion by roughly a factor of two, comparable to the dispersion measured recently in a normal galaxy at z=1. This comparison suggests that the mass and star formation rate surface densities may be similar in galaxies from z=0-1 and that the high star formation rates seen at z=1 may be partly due to the presence of physically larger molecular gas disks.
We have made the first map of CO(J=3-2) emission covering the disk of the edge-on galaxy, NGC~4631, which is known for its spectacular gaseous halo. The strongest emission, which we model with a Gaussian ring,occurs within a radius of 5 kpc. Weaker d
isk emission is detected out to radii of 12 kpc, the most extensive molecular component yet seen in this galaxy. From comparisons with infrared data, we find that CO(J=3-2) emission more closely follows the hot dust component, rather than the cold dust,consistent with it being a good tracer of star formation. The first maps of $R_{3-2/1-0}$, H$_2$ mass surface density and SFE have been made for the inner 2.4 kpc radius region. Only 20% of the SF occurs in this region and excitation conditions are typical of galaxy disks, rather than of central starbursts. The SFE suggests long gas consumption timescales ($>$ $10^9$ yr). The velocity field is dominated by a steeply rising rotation curve in the region of the central molecular ring followed by a flatter curve in the disk. A very steep gradient in the rotation curve is observed at the nucleus, providing the first evidence for a central concentration of mass: M$_{dyn},=,5,times,10^7$ M$_odot$ within a radius of 282 pc. The velocity field shows anomalous features indicating the presence of molecular outflows; one of them is associated with a previously observed CO(J=1-0) expanding shell. Consistent with these outflows is the presence of a thick ($z$ up to $1.4$ kpc) CO(J=3-2) disk. We suggest that the interaction between NGC~4631 and its companion(s) has agitated the disk and also initiated star formation which was likely higher in the past than it is now. These may be necessary conditions for seeing prominent halos.
We use Herschel Space Observatory data to place observational constraints on the peak and Rayleigh-Jeans slope of dust emission observed at 70-500 microns in the nearby spiral galaxy M81. We find that the ratios of wave bands between 160 and 500 micr
ons are primarily dependent on radius but that the ratio of 70 to 160 micron emission shows no clear dependence on surface brightness or radius. These results along with analyses of the spectral energy distributions imply that the 160-500 micron emission traces 15-30 K dust heated by evolved stars in the bulge and disc whereas the 70 micron emission includes dust heated by the active galactic nucleus and young stars in star forming regions.
M82 is a unique representative of a whole class of galaxies, starbursts with superwinds, in the Very Nearby Galaxy Survey with Herschel. In addition, its interaction with the M81 group has stripped a significant portion of its interstellar medium fro
m its disk. SPIRE maps now afford better characterization of the far-infrared emission from cool dust outside the disk, and sketch a far more complete picture of its mass distribution and energetics than previously possible. They show emission coincident in projection with the starburst wind and in a large halo, much more extended than the PAH band emission seen with Spitzer. Some complex substructures coincide with the brightest PAH filaments, and others with tidal streams seen in atomic hydrogen. We subtract the far-infrared emission of the starburst and underlying disk from the maps, and derive spatially-resolved far-infrared colors for the wind and halo. We interpret the results in terms of dust mass, dust temperature, and global physical conditions. In particular, we examine variations in the dust physical properties as a function of distance from the center and the wind polar axis, and conclude that more than two thirds of the extraplanar dust has been removed by tidal interaction, and not entrained by the starburst wind.
We used 3.6, 8.0, 70, 160 micron Spitzer Space Telescope data, James Clerk Maxwell Telescope HARP-B CO J=(3-2) data, National Radio Astronomy Observatory 12 meter telescope CO J=(1-0) data, and Very Large Array HI data to investigate the relations am
ong PAHs, cold (~20 K) dust, molecular gas, and atomic gas within NGC 2403, an SABcd galaxy at a distance of 3.13 Mpc. The dust surface density is mainly a function of the total (atomic and molecular) gas surface density and galactocentric radius. The gas-to-dust ratio monotonically increases with radius, varying from ~100 in the nucleus to ~400 at 5.5 kpc. The slope of the gas-to-dust ratio is close to that of the oxygen abundance, suggesting that metallicity strongly affects the gas-to-dust ratio within this galaxy. The exponential scale length of the radial profile for the CO J=(3-2) emission is statistically identical to the scale length for the stellar continuum-subtracted 8 micron (PAH 8 micron) emission. However, CO J=(3-2) and PAH 8 micron surface brightnesses appear uncorrelated when examining sub-kpc sized regions.
